1. Field of the Invention
This invention relates to a polymer optical waveguide with a buffer layer and a method of making the polymer optical waveguide.
2. Description of the Related Art
In recent years, polymer optical waveguides made of a polymer material are researched and developed since they are advantageous in workability and manufacturing cost as compared to glass optical waveguides made of a glass material.
Various polymer materials for a waveguide main body formed on a substrate are suggested, for example, acrylic (See Japanese patent application laid-open No. 10-170739), epoxy (See Japanese patent application laid-open No. 2002-286953), and polyimide (See Japanese patent application laid-open No. 2003-215364). There is a worry that these materials are disadvantageous in heat or moisture resistance since they can be subjected to a variation in refractive index or optical loss under high temperature and high humidity.
When a glass transition temperature of the waveguide main body falls within the working temperature range, the refractive index may be much more varied at a temperature higher than the glass transition temperature. In this case, the optical characteristics of the polymer optical waveguide will be affected more badly. Therefore, a material with a high glass transition temperature is generally used for the polymer waveguide main body.
On the other hand, Japanese patent application laid-open No. 2004-126399 (hereinafter referred to as '399) discloses a glass optical waveguide that is provided with a clad layer (made of glass such as TiO2) for buffering a thermal stress with a linear expansion coefficient intermediate between its substrate and waveguide main body so as to reduce the thermal stress accumulated in the glass optical waveguide in changing the temperature to make the waveguide temperature-independent.
Further, Japanese patent application laid-open No. 2002-189138 discloses a method of a polymer optical waveguide that a stress generating film of metal is formed on the back of a substrate such that the substrate is previously bent by the stress generating film, and a waveguide main body is then formed on the surface of the substrate so as not to have an internal stress remained.
However, the polymer optical waveguide using a material with a high glass transition temperature has problems that the waveguide main body may be peeled off from the polymer waveguide main body or cracked when subjected to a heat-shock test (e.g., a test that the surrounding temperature is repeatedly increased and decreased in the temperature range of −40 to 85° C.) since where the waveguide main body is formed directly on the substrate.
Further, an optical multiplexer fabricated using such a polymer optical waveguide must have a variation in optical output when the surrounding temperature is changed.
The clad layer (made of glass such as TiO2) of '399 concerning the glass optical waveguide is effective when a difference in linear expansion coefficient between the substrate and the waveguide main body is small. However, incase of the polymer optical waveguide, since the linear expansion coefficient of the waveguide main body is much greater than that of a typical Si wafer or silica glass substrate, the difference in linear expansion coefficient between the substrate and the waveguide main body must be big.
Therefore, if the clad layer of '399 is directly used for the polymer optical waveguide, the buffering effect will be insufficient. In order to have the sufficient buffering effect, the thickness must be much increased. Further, since the clad layer of '399 is made of glass, it is difficult to process as compared to resins and therefore the manufacturing cost increases.